Genetic factors contribute to the expression of autoimmune diseases in both mice and humans. These genes are largely uncharacterized; moreover, their gene products and mechanisms of action are unknown. A recessive mutation, lpr, that occurred in MRL mice results in profound lymphadenopathy with expansion of an unusual subpopulation of T cells and generalized autoimmune disease. A "reverse" genetic approach is proposed to identify the mutant or deficient gene products of the lpr locus and MRL genes that interact with lpr to result in renal disease. The method relies on defining the physical location of these genes. RFLV and microsatellite polymorphisms will be used as markers in two different crosses segregating for the recessive genes that are critical to clinical and serologic disease in these mice. The renal histology will be scored and correlated with the genotype analyses and autoantibody profiles. The localization of lpr to mouse Chr 19 will be confirmed and saturation mapping using clones from a flow sorted mouse Chr 19 library will be performed. Of most interest will be defining the MRL quantitative trait loci (QTL) that predispose to nephritis. Detailed mapping of each relevant region of the genome will be undertaken using available clones localized to the specific chromosome segment and clones likely to be in that region of the genome. If necessary, a subchromosome specific library or a chromosome microdissection library will be generated to allow saturation mapping of one of the identified QTL regions. Long range restriction site analyses of the relevant regions of the mouse genome will be performed. If, as is likely, one or more autoimmune loci are within a chromosomal segment largely conserved between the mouse and human genome, these studies may identify the chromosomal location of human "autoimmune genes". A variety of strategies including the use of chromosome jumping and a mouse yeast artificial chromosome library, in addition to long range restriction mapping will be used to more precisely define the molecular location of the critical gene (s) responsible for genetic predisposition. Comparison with MRL-plus-minus/plus-minus mice will be useful for determining the lpr mutation as will comparison of CBA/KJ with a new mutation at the lpr locus in a strain designated CBA/KJ-lpr(cg) . For other localized "autoimmune genes", candidate genes may be suggested by their chromosomal location. Definition of a single gene that predisposes to autoimmunity should allow fundamental insight into molecular basis for disease in mice and humans.